So, considering I already have a one hander, should I just skip the switch and go straight to a spey? Should I go with a longer spey to begin with, or should I stick around 13ft?

I would get the Spey rod that you will want to use for the fish that you are targeting. With today's newer styles of fly lines, even cavemen can learn how to Spey cast. Read Simon's Gawesworth's Understanding Spey Lines 2010 on the different styles of lines. In my opinion, the easiest line to work with is the Skagit style.

For all of my Northern California and Southern Oregon fishing, I use a 12'6" 6 weight. You can go to a 13' or 13'6" 7 weight rod if you want to really dredge with super heavy flies (lead wrapped, dumb bell eyed). To me the difference between a 6 and 7 weight is the kind of delivery system that the caster wants.